Lithographic apparatus and device manufacturing method incorporating a pressure shield

ABSTRACT

A lithographic apparatus is disclosed that has a movable article support configured to hold and move an article, a radiation control system configured to control a beam of radiation to be targeted onto the article, the article support, or both, the article to be moved relatively to the radiation control system by the movable article support for measurement, exposure, or both purposes, and a pressure shield that is mechanically uncoupled from the radiation control system to shield against pressure waves induced by the article support so as to help prevent displacement of the radiation control system caused by the pressure waves.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.11/314,156, filed Dec. 22, 2005(that issued a U.S. Pat. No. 7,492,441 onFeb. 17, 2009), which is incorporated herein by reference in itsentirety.

BACKGROUND

1. Field

The present invention relates to a lithographic apparatus.

2. Background

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In a lithographic apparatus, an article, such as a substrate, a mask ora fiducial to be placed in the radiation beam, is held by an articlesupport member, for example, by a clamping electrode, vacuum suction orotherwise.

Typically, these articles are positioned in a single or multistagepositioning procedure which includes alignment and/or height adjustmentof the article. To this end, typically one or more contactlessmeasurement systems are used, which measurement systems direct aradiation beam to the article for measurement purposes, e.g. byinterferometric measurements or by the use of a grating image. Also, aprojection system is typically used to transfer a patterned beam onto atarget portion of the article, in particular, the substrate, forexposure purposes of the article. For all these radiation controlsystems, in particular, a positioning sensor system and/or a projectionsystem, a desire arises for them to be very stably mounted in order forthem to be able to provide measurements and/or exposed features with inthe order of (sub-) nanometer accuracies. To this end, a conventionallithographic apparatus may provide a metrology frame used to hold andkeep substantially in place equipment used to measure distances used toposition the substrate and the like and/or to hold and keepsubstantially in place equipment used to transfer a pattern to thesubstrate. This metrology frame is a stabilized frame that is protectedagainst vibrations from outside so that it creates a super stableenvironment or “silent world” wherein the lithographic process iscarried out, in particular, the positioning of a substrate and transferof a pattern onto a target portion of the substrate.

On the other hand, a lithographic apparatus has moving parts whichincreasingly move faster and faster (to increase throughput) and whichput a burden on the stability of equipment in the lithographicapparatus, especially on the mounting of measurement and/or exposureequipment.

BRIEF SUMMARY

An aspect that should be taken into account is the generation ofpressure waves by moving parts. These waves can be problematic, forexample, for interferometric measurement in that the refractive index ofthe gas through which the interferometric beam passes may be altered byvariation of pressure, so that a measurement becomes inaccurate. Inaddition or alternatively, with the increasing faster movement of theseparts, these pressure waves may cause a physical displacement of aradiation control system, which may give rise to deteriorated imagingand/or measurement accuracy. Accordingly, it would be desirable, forexample, to provide a lithographic apparatus which is more robust andless sensitive to the presence of these pressure waves.

According to an aspect of the invention, there is provided alithographic apparatus, comprising: a movable article support configuredto hold and move an article; a radiation control system configured tocontrol a beam of radiation to be targeted onto the article, the articlesupport, or both, the article to be moved relatively to the radiationcontrol system by the movable article support for measurement, exposure,or both purposes; and a pressure shield that is mechanically uncoupledfrom the radiation control system to shield against pressure wavesinduced by the article support so as to help prevent displacement of theradiation control system caused by the pressure waves.

According to an aspect of the invention, there is provided alithographic apparatus configured to transfer a pattern to a substrate,comprising: a table configured to be moveable and to hold a substrate ora patterning device; and a pressure shield that is not directlyconnected to a displacement sensitive device to shield the device frompressure waves induced by movement of the table so as to substantiallyprevent displacement of the device caused by the pressure waves.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising: moving a table holding a substrate ora patterning device; shielding a displacement sensitive device frompressure waves induced by the movement of the table so as tosubstantially prevent displacement of the device caused by the pressurewaves; and transferring a pattern to a substrate.

In the context of this application, the “article” may be a substrate(e.g., a wafer), a patterning device (e.g., a reticle or mask), anyother article or optical element (e.g., a mask blank) that is providedin the radiation path of a radiation control system, or any combinationof the foregoing. More specifically terms such as a substrate to beprocessed in manufacturing devices employing lithographic projectiontechniques, a lithographic projection mask in a lithographic projectionapparatus, a mask handling apparatus such as a mask inspection orcleaning apparatus, or a mask manufacturing apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Embodiments of the invention will not be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention.

FIG. 2 schematically shows a conventional setup for a lithographicapparatus, wherein no pressure shield is applied according to anembodiment of the invention.

FIG. 3 illustrates a top view of a simulated pressure profile in theambient environment around the tables 2 and 3 depicted in FIG. 2.

FIG. 4 schematically shows a first embodiment of the invention,illustrating a pressure shield having through holes.

FIG. 5 schematically shows a second embodiment of the invention,illustrating a substantially transparent pressure shield.

FIG. 6 schematically shows a third embodiment of the invention,illustrating a pressure shield for a projection system and an alignmentsystem; and

FIG. 7 shows a fourth embodiment of the invention, illustrating afurther shielding arrangement.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or EUV radiation);

a support structure (e.g. a mask table) MT constructed to hold apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF2 (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioner PM and another position sensor IF1 canbe used to accurately position the patterning device MA with respect tothe path of the radiation beam B, e.g. after mechanical retrieval from amask library, or during a scan. In general, movement of the supportstructure MT may be realized with the aid of a long-stroke module(coarse positioning) and a short-stroke module (fine positioning), whichform part of the first positioner PM. Similarly, movement of thesubstrate table WT may be realized using a long-stroke module and ashort-stroke module, which form part of the second positioner PW. In thecase of a stepper (as opposed to a scanner) the support structure MT maybe connected to a short-stroke actuator only, or may be fixed.Patterning device MA and substrate W may be aligned using patterningdevice alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice MA, the patterning device alignment marks may be located betweenthe dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 shows a side view of a portion of a conventional setup of alithographic apparatus, in particular, a multiple table setup 1, e.g.,two or more substrate tables. In the shown embodiment, two tables 2, 3are used in parallel, wherein on one table 3, preparatory steps may becarried out, such as measurement of a substrate 4, while the other table2 is used for exposure purposes of another substrate 5, e.g., receivinga patterned beam from a projection system (not shown). As is shownschematically by waves 6, the moving table 2 (the movement indicatedschematically by arrow P) creates pressure waves that impinge on theother table 3 and the measurement unit 7 in the vicinity thereof. ArrowsQ show that most, if not all, of the bottom area of the measurement unitis hit by the pressure wave, causing a relatively large displacementforce on the measurement unit 7. Typically, such a measurement unit 7 isused to carry out an alignment step and/or to create a height map of asubstrate. Specifically, for alignment, the measurement unit 7 comprisesan alignment module 8 configured to measure in plane positions of asubstrate. For level sensing, the measurement unit 7 comprises a levelsensor 9 configured to measure out of plane positions of a substrate.

In an embodiment, the measurement unit 7 is an example of a radiationcontrol system. The measurement unit 7 comprises typically a lasersource or the like (not shown) that provides a radiation beam 10 to beprojected on the substrate 4, reflected by the substrate 4, and measuredin the unit 7. A radiation control system encompasses systems whichcontrol radiation, irrespective whether the radiation is produced in thesystem or only relayed from an outside source. Furthermore, theradiation control system typically both emits and receives radiation butmay only emit or receive radiation.

As already indicated, the moving table 2, especially the accelerationthereof, creates pressure waves that impinge on the measurement unit 7.Where the unit 7 must operate with (sub-) nanometer accuracy, thesepressure waves can provide displacements which may be deleterious.

FIG. 3 illustrates a top view of a simulated pressure profile 11 in theambient environment around the tables 2 and 3. Pressure fluctuations inthe ambient environment during acceleration can be estimated, assuming aID problem in which a column of gas with a typical length L has to beaccelerated: ΔP_(acc)≈LρdV/dt, where ρ is the gas density and typicalaccelerations dV/dt of >10 m/s². Furthermore, constant velocities mayinduce a pressure fluctuation due to gas which is moving away in frontof the table that can be estimated to be 1 to 2 times the stagnationpressure which is: ΔP_(stag)≈½ ρV².

A pressure profile 11 was calculated at t=0.04 sec where table 2 isaccelerated from 0 to 0.48 m/s with dV/dt=12 m/s² using computationalfluid dynamics. Contour lines show a pressure difference of 1.6 Pa. FromFIG. 3, it can be seen that a variation of several Pa pressure force ispossible. With an assumption of an effective area of (0.1×0.1) m²,displacements in the order of several nanometers are possible even forrelatively stiff mountings 12, having a stiffness of 1E7 N/m, holdingthe measurement unit 7. Such displacement can be distributed integrallyover the measurement unit 7 and need not be concentrated in themountings 12 shown in FIG. 2.

In FIG. 4 a first embodiment of the invention is shown. Here, themeasurement unit 7 is provided with a pressure shield 13 mounted on themetrology frame 14. The pressure shield 13 provides passage to the oneor more radiation beams 10 of the measurement system 7 so thatcontactless measurement of the substrate 4 may be carried outunhindered. However, the pressure shield 13 blocks or at leastsubstantially reduces the pressure waves 6 from reaching the measurementunit 7. In this way displacement of the measurement system caused by thepressure waves can be suppressed. In an embodiment, the pressure shield13 is mechanically uncoupled from the measurement system 7. Typically,this means that no direct mechanical coupling to the measurement systemis present but only indirect coupling, for instance, via metrology frame14. However, a direct coupling may be used that is very elastic, so thatthe wave energy is lost in the elastic coupling without displacing themeasurement unit 7. Since the metrology frame 14 has a high mass andstiffness, local pressure variations are damped in the shield 13 andsubstantially do not reach the measurement unit 7. Alternatively, theshield may be completely uncoupled from the measurement system, forinstance, by providing a shield that is mounted to an external staticframe (not shown) instead of the metrology frame 14 that is used forholding the radiation control system 7.

FIG. 4 schematically shows a pressure shield 13 that is mounted directlyon the metrology frame 14 via mounting 15, which comprises through holes16. The radiation beam 10 can pass through the holes 16. Since thethrough holes 16 are relatively small in width, only a limited fractionof the pressure waves reach the measurement unit 7 and do not provide asignificant displacement of the measurement unit 7.

In the embodiment of FIG. 5, the pressure shield 13 comprises atransparent plate 17 having low or zero optical power. In thisembodiment, the measurement unit 7 is completely shielded from thepressure waves 6 and the transparent plate 17 transmits the beam 10unhindered. If the plate 13 is substantially flat, a vertical orhorizontal displacement thereof due to an impinging pressure wave haslittle or no impact on the measurement results of the measurement unit7. Where the plate has a weak tilt, some errors may be introduced whichcan be dealt with by stiff pressure shield mountings 15.

In FIG. 6, another embodiment is schematically shown, wherein theshielding is applied to a projection system 18 as an example of aradiation control system. Here, also, the pressure waves may put stresson the mountings 19 of the projection system, which may give rise tounwanted displacements. Accordingly, a pressure shield 20 is providedthat is mechanically uncoupled from the projection system 18 to helpshield the projection system 18 against pressure waves 6 induced by oneor more article supports 2, 3 so as to help prevent displacement of theprojection system caused by the pressure waves 6. The pressure shield 20is mounted to the metrology frame 14 through mountings 21 and followsthe shape of the projection system 18 (although it does not need to). Ithas a transparent plate 17 to provide passage to the imaging beam of theprojection system 18. Hence the lowest optical part of the projectionsystem 18 is formed by a flat plate or low power element that is mountedon the metrology frame 14. Alternatively, a small opening for theradiation path can be used, possibly in combination with a purge hood oran immersion structure for immersion lithography purposes.

In FIG. 7, a fourth embodiment is schematically shown of a shieldingarrangement of a lithographic apparatus. In particular, in thisarrangement, the substrate table setup 1 is entirely or significantlyshielded in a compartment 22 which is accessible by the radiationcontrol systems 7 and 18 (e.g., projection system 18 and measurementunit 7) via transparent plates 17 or small through holes mounted in theshield 22. In this arrangement barometric pressure correction can becarried out for instance, by actively controlling the barometricpressure in the compartment 22. Accordingly, gas pressure variation andcorresponding refractive index changes may also be slowed down.

Although the invention has been described with reference to theembodiments described herein, the skilled person will readily appreciatethat variations and modifications to these embodiments are possiblewithout departing from the scope of the claims. Specifically, theradiation control system can be of other nature than the shownprojection, alignment and/or leveling systems. For instance, such aradiation control system may comprise an interferometric measurementsystem used for in plane measurement of the tables 2, 3 shown in FIG. 2.Also, the type of radiation is not of significant importance and may bethe radiation used to expose the substrate (i.e., to transfer a patternto the substrate) or can be of other wavelengths, for instance, visiblelight or other type of suitable electromagnetic waves. These variationsand modifications are deemed to be comprised in the scope of the claimsbelow. Furthermore, although specific reference may be made in this textto the use of lithographic apparatus in the manufacture of ICs, itshould be understood that the lithographic apparatus described hereinmay have other applications, such as the manufacture of integratedoptical systems, guidance and detection patterns for magnetic domainmemories, flat-panel displays, liquid-crystal displays (LCDs), thin-filmmagnetic heads, etc. The skilled artisan will appreciate that, in thecontext of such alternative applications, any use of the terms “wafer”or “die” herein may be considered as synonymous with the more generalterms “substrate” or “target portion”, respectively. The substratereferred to herein may be processed, before or after exposure, in forexample a track (a tool that typically applies a layer of resist to asubstrate and develops the exposed resist), a metrology tool and/or aninspection tool. Where applicable, the disclosure herein may be appliedto such and other substrate processing tools. Further, the substrate maybe processed more than once, for example in order to create amulti-layer IC, so that the term substrate used herein may also refer toa substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that an embodiment of the invention may be used inother applications, for example imprint lithography, and where thecontext allows, is not limited to lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of imprintable medium supplied to the substrate whereupon themedium is cured, for example, by applying electromagnetic radiation,heat, pressure or a combination thereof. The patterning device is movedout of the medium leaving a pattern in it after the medium is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm). Theterm “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. For example, an embodiment of the inventionmay take the form of a computer program containing one or more sequencesof machine-readable instructions describing a method as disclosedherein, or a data storage medium (e.g. semiconductor memory, magnetic oroptical disk) having such a computer program stored therein. Thus, itwill be apparent to one skilled in the art that modifications may bemade to the invention as described without departing from the scope ofthe claims set out below.

1. A lithographic apparatus, comprising: a substrate support configuredto hold a substrate; a projection system configured to control apatterned beam of radiation to be targeted onto the substrate, thesubstrate to be moved relative to the projection system by the substratesupport for measurement, exposure, or both purposes; a pressure shieldcomprising a substantially transparent plate that is mechanicallyuncoupled from the projection system to shield against pressure wavesinduced by the substrate support so as to help prevent displacement ofthe projection system caused by the pressure waves and to providepassage of the patterned beam of radiation to the substrate; and animmersion structure configured to supply immersion liquid between theprojection system and the substrate during exposure.
 2. The lithographicapparatus according to claim 1, wherein the pressure shield comprises aflat plate.
 3. The lithographic apparatus according to claim 2, whereinthe pressure shield comprises through holes.
 4. The lithographicapparatus according to claim 2, wherein the substantially transparentplate has low or zero optical power.
 5. The lithographic apparatusaccording to claim 1, wherein the substantially transparent plate ismounted to a metrology frame that is shared by the projection system. 6.The lithographic apparatus according to claim 1, wherein thesubstantially transparent plate is mounted to a static frame notdirectly shared by the projection system.
 7. The lithographic apparatusaccording to claim 1, wherein the substrate support is substantiallyshielded from the projection system by the pressure shield.
 8. Thelithographic apparatus according to claim 1, wherein the projectionsystem comprises a measurement system configured to project ameasurement beam onto a portion of the substrate, the substrate supportor both or to detect a measurement beam traveling from a portion of thesubstrate, the substrate support, or both.
 9. The lithographic apparatusaccording to claim 8, wherein the measurement system comprises analignment system, a level sensor system, or both.
 10. The lithographicapparatus according to claim 8, wherein the measurement system comprisesan interferomeVic measurement system.
 11. The lithographic apparatusaccording to claim 1, further comprising an article support configuredto hold a patterning device, the patterning device configured to imparta beam of radiation with a pattern in its cross-section to produce thepatterned beam of radiation.